Parasitism is an aspect of Interspecies interaction, a type of symbiotic relationship between organisms of different species in which one, the parasite, benefits from a prolonged, close association with the other, the host, which is harmed. In general, parasites are much smaller than their hosts, show a high degree of specialization for their mode of life and reproduce more quickly and in greater numbers than their hosts. Classic examples of parasitism include the interactions between vertebrate hosts and such diverse animals as the tapeworms, flukes, Plasmodium species and fleas.

The harm and benefit in parasitic interactions concern the biological fitness of the organisms involved. Parasites reduce host fitness in many ways, ranging from general or specialized pathology (such as castration), impairment of secondary sex characteristics, to the modification of host behaviour. Parasites increase their fitness by exploiting hosts for food, habitat and dispersal.

Although the concept of parasitism applies unambiguously to many cases in nature, it best considered part of continuum of types of interactions between species, rather than an exclusive category. Particular interactions between species may satisfy some but not all parts of the definition. In many cases, it is difficult to demonstrate the host is harmed. In others, there may be no apparent specialization on the part of the parasite, or the interaction between the organisms may be short-lived. For example, because of the episodic nature of its feeding habits, the mosquito is not considered parasitic. In medicine, only eukaryotic organisms are considered parasites, to the exclusion of bacteria and viruses. Some branches of biology, however, do regard members of these groups to be parasitic.

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Parasites are classified based on a variety of aspects of their interactions with their hosts and on their life cycles.

Those that live inside the host are called endoparasites (e.g., hookworms) and those that live on its surface are called ectoparasites (e.g., some mites).

An epiparasite is a one that feeds on another parasite. This relationship is also sometimes referred to as "hyperparasitism", and is most common in the insect world. For example, some wasp or flylarvae are endoparasites of ichneumon wasp larvae, which in turn parasitize wood-boring beetle larvae.

Parasitoids are parasites that eventually cause the host to die as a result of infection, often from a loss of host tissues or nutrients. Parasitoids are sometimes called necrotrophs. In contrast, biotrophic parasites cannot survive in a dead host.

Social parasites take advantage of interactions between members of social organisms such as ants or termites. In kleptoparasitism, parasites appropriate food gathered by the host. An example is the brood parasitism practiced by many species of cuckoo. Many cuckoos use other bird species as "babysitters", depositing their eggs in the nest of the host species, which raise the cuckoo young as one of their own.

Parasitism can take the form of isolated cheating or exploitation among more generalized mutualistic interactions. For example, broad classes of plants and fungi exchange carbon and nutrients in common mutualistic mycorrhizal relationships; however, a few plants species (known as myco-heterotrophs) "cheat" by taking carbon from a fungus rather than donating it.

Biotrophic parasitism is an extremely common mode of life that has arisen independently many times in the course of evolution. Depending on the definition used, as many as half of all animals have at least one parasitic phase in their life cycles[1], and it is also frequent in plants and fungi. Moreover, almost all free-living animals are host to one or more parasite taxa[2].

Parasites evolve in response to defense mechanisms of their hosts. Examples of host defenses include the toxins produced by plants to deter parasitic fungi and bacteria, the complex vertebrateimmune system, which can target parasites through contact with bodily fluids, and behavioural defenses. An example of the latter is the avoidance by sheep of open pastures during spring, when roundworm eggs accumulated over the previous year hatch en masse. As a result of these and other host defenses, some parasites evolve adaptations that are specific to a particular host taxon and specialize to the point where they infect only a single species. Such narrow host specificity can be costly over evolutionary time however, if the host species becomes extinct. Thus, many parasites are capable of infecting a variety of host species that are more or less closely related, with varying success.

Host defenses also evolve in response to attacks by parasites. Theoretically, parasites may have an advantage in this evolutionary arms race because of their more rapid generation time. Host reproduce less quickly than parasites, and therefore have fewer chances to adapt than their parasites do over a given span of time.

In some cases, a parasite species may coevolve with its host taxa. In theory, long-term coevolution should lead to a relatively stable relationship tending to commensalism or mutualism, in that it is in the evolutionary interest of the parasite that its host thrive. For example, although animals infected with parasitic worms are often clearly harmed, and therefore parasitized, such infections may also reduce the prevalence and effects of autoimmune disorders in animal hosts, including humans[3].

The presumption of a shared evolutionary history between parasites and hosts can sometimes elucidate how host taxa are related. For instance, there has been dispute about whether flamingos are more closely related to the storks and their allies or to ducks, geese and their relatives. The fact that flamingos share parasites with ducks and geese is evidence these groups may be more closely related to each other than either is to storks.

Parasitism is part of one explanation for the evolution of secondary sex characteristics seen in breeding males throughout the animal world, such as the plumage of male peacocks and manes of male lions. According to this theory, female hosts select males for breeding based on such characteristics because they indicate resistance to parasites and other disease.

Because they are small and often hidden from view, parasites are often ignored in ecology. However, parasites are ubiquitous and play important roles in ecosystems and, considered in their own right, pose unique problems in ecology. More recently, therefore, parasite ecology has matured as a discipline and begun to integrate with ecology in the broader sense.

Hosts represent discrete habitat patches that can be occupied by parasites. A hierarchical set of terminology has come into use to describe parasite assemblages at different host scales.

An infrapopulation is all the parasites of one species in a single individual host

A metapopulation is all the parasites of one species in a host population

An infracommunity is all the parasites of all species in a single individual host

A component community all the parasites of all species in a host population

A compound community is all the parasites of all species in all host species in an ecosystem.

The diversity ecology of parasites differs markedly from that of free-living organisms. That is, the determinants of species richness and relative abundance animals. For free-living organisms, diversity ecology features many strong conceptual frameworks including Macarthur and Wilson's theory of island biogeography, Diamond's assembly rules and, more recently, null models such as Hubbell's neutral theory of biodiversity and biogeography. Frameworks are not so well developed for parasites and in many ways they do not fit the free-living models. For example, island biogeography is predicated on fixed spatial relationships between habitat patches ("sinks"), usually with reference to a mainland ("source"). Parasites inhabit hosts, which represent mobile habitat patches with dynamic spatial relationships. There is no true "mainland" other than the sum of hosts (host population); in this way parasite component communities in host populations are metacommunities.

Nonetheless, different types of parasite assemblages have been recognised in host individuals and populations, and many of the patterns observed for free-living organisms are also pervasive among parasite asemblages. The most prominent of these is the interactive-isolationist continuum. This proposes that parasite assemblages occur along a cline from interactive communities, where niches are saturated and interspecific competition is high, to isolationist communities, where there are many vacant niches and interspecific interaction is not as important as stochastic factors in providing structure to the community. Whether this is so, or whether community patterns simply reflect the sum of underlying species distributions (no real "structure" to the community), has not yet been established.

Parasites inhabit living organisms, and as a result face problems that free-living organisms do not. Hosts, the only habitats in which parasites can survive, actively try to avoid, repel and destroy parasites. Parasites employ numerous strategies for getting from one host to another, a process sometimes referred to as parasite transmission, or the colonization of new hosts.

Many endoparasites infect their host by penetrating its external surface, while others must be ingested by the host. Once inside the host, adult endoparasites need to shed offspring into the external environment in order to infect other hosts. Many adult endoparasites reside in the host’s gastrointestinal tract, where offspring can be shed along with host excreta. Adult stages of tapeworms, thorny-headed worms and most flukes use this method.

Larval stages of endoparasites often infect sites in the host other than the blood or gastrointestinal tract. In many such cases, larval endoparasites require their host to be consumed by the next host in parasite’s life cycle in order to survive and reproduce. Alternatively, larval endoparasites may shed free-living transmission stages that migrate through the host’s tissue into the external environment, where they actively search for or await ingestion by other hosts. The foregoing strategies are used, variously, by larval stages of tapeworms, thorny-headed worms, flukes and parasitic roundworms.

Many ectoparasites, such as monogenean worms, rely on direct contact between hosts to colonize new hosts, but other methods are also used. Ectoparasitic arthropods may rely on host-host contact (e.g., many lice), shed eggs that survive off the host (e.g., fleas), and/or wait in the external environment for an encounter with a host (e.g., ticks). Some aquatic leeches locate hosts by sensing movement and only attach when certain temperature and chemical cues are present.

Some parasites modify host behaviour to make transmission to other hosts more likely. For example, in California salt marshes, the fluke Euhaplorchis californiensis reduces the ability of its killifish host to avoid predators [4]. This parasite matures in egrets, which are more likely to feed on infected killifish than on uninfected fish. Another example is the protozoan Toxoplasma gondii, a parasite that matures in cats but can be carried by many other mammals. Uninfected rats avoid cat odours, but rats infected with T. gondii are drawn to this scent, a change which may increase transmission to feline hosts[5].

Modifying the behaviour of infected hosts to make transmission to other hosts more likely is one way parasites can affect the structure of ecosystems. For example, in the case case of Euhaplorchis californiensis discussed above, it is plausible that the abundance of local predator and prey species would be different if this parasite were absent from the system.

Although parasites are often omitted in depictions of food webs, they usually occupy the top position. Parasites can function like keystone species, reducing the dominance of superior competitors and allowing competing species to co-exist.

Many parasites require multiple hosts of different species to complete their life cycles and rely on predator-prey or other stable ecological interactions to get from one host to another. In this sense, the parasites in an ecosystem reflect the “health” of that system. For example, in a typical life cycle for the group, a fluke may infect first a snail, then a fish, then a piscivorousbird. Transmission only occurs if infected snails are in sufficient proximity with fish, which in turn must be consumed by the bird host, which must defecate, shedding fluke eggs, sufficiently close to the snails. Parasites cannot survive without the “functioning” of ecological links such as these.

Parasitic diseases account for a large proportion of human morbidity and mortality, and doubtless contribute significantly to morbidity and mortality among all animal populations as well. In this sense, parasitic disease is an important ecological force shaping the biosphere.